However, a more sensitive probe of the EoS is given by the speed of sound squared:
\(c_s^2 = \partial p/\partial e\) . Free, massless fermions
have \(c_s^2 = 1/3\) , which is termed soft. In contrast, several studies of neutron
stars find that it is essential for the nuclear EoS to have a region in which the EoS is stiff,
where \(c_s^2\)  significantly larger than \(1/3\) 
[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}.
